The present invention is directed to a method for increasing the yield of light olefins during the conversion of oxygenates to olefins by converting heavy hydrocarbons in the product to light olefins in a separate auxiliary reactor.
Light olefins (defined herein as xe2x80x9cethylene and propylenexe2x80x9d) serve as feeds for the production of numerous chemicals. Light olefins traditionally are produced by petroleum cracking. Because of the limited supply and/or the high cost of petroleum sources, the cost of producing olefins from petroleum sources has increased steadily.
Alternative feedstocks for the production of light olefins are oxygenates, such as alcohols, particularly methanol, dimethyl ether, and ethanol. Alcohols may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for olefin production.
Because light olefins are the most sought after products of such a reaction, a continuing need exists for new methods to increase the yield of light olefin products and reduce the yield of unwanted products, such as xe2x80x9cheavyxe2x80x9d hydrocarbons having molecular weights heavier than propane.
The present invention provides a method for increasing light olefin yield during conversion of oxygenates to olefins. The method comprises: contacting an oxygenate feed in a primary reactor with a non-zeolitic molecular sieve catalyst under first conditions effective to produce a first product comprising light olefins; separating the first product into light olefins and a heavy hydrocarbon fraction; feeding the heavy hydrocarbon fraction either back to the primary reactor or to a separate auxiliary reactor; and, subjecting the heavy hydrocarbon fraction to second conditions effective to convert at least a portion of the heavy hydrocarbons to light olefins.